This invention relates to a process for producing a stiff board of cement-like material reinforced by fibrous material, having a high density, and to an apparatus for carrying out the process.
Heretofore, a dense stiff board of cement-like material reinforced by fibrous material has been prepared by mixing fibrous material with powdery cement-like material or a slurry of cement-like material with vigorous stirring and forming the mixture into the desired shape by means of various techniques. However, according to the conventional process, in the mixing step, the fibrous material and the powdery cement-like material or the slurry must be stirred mechanically to a great extent, and as a result of this mechanical stirring the fibrous material is damaged. Further according to the conventional process, superfluous water must be removed, but it is difficult to lower the water content of cement cake to a predetermined amount, that is, an amount stoichiometrically required for hydration. The strength of cement-like material after hydration is in inverse proportion to the water content of the cement-like material mixture to be cured. Consequently it is preferred to make the water content of the cement-like mixture as low as possible while still satisfying the amount stoichiometrically required for hydration.
Generally, mixing of a solid with a small amount of liquid or viscous liquid, or mixing of highly viscous liquids is referred to as "kneading". Since very viscous materials require a great deal of energy to mix them together, the operation is more difficult than that of the usual solid mixing or liquid mixing, and therefore it is often carried out with a relatively small apparatus and in a batch-wise system. Consequently, in using the kneading process, fibrous material is badly damaged and it is not satisfactory dispersed. In the kneading process, water is added while the powdery cement-like material and the fibrous material are being mixed, but the amount of fibrous material incorporated is restricted to less than 3% and the length of the fibrous material used is also restricted to less than 15 mm.
Various types of mixers or kneading machines can be used for kneading cement-like materials and fibrous materials, but most of them are batch-wise systems and their capacities are relatively small although the power generated by them is large.
In preparing an asbestos-cement plate, a large amount of water is employed to carry out the mixing of the asbestos and cement in a slurry state so that fibers of the asbestos are uniformly dispersed in the cement. However, in such a process, it is necessary to remove superfluous water at a subsequent stage, and it is difficult to prepare a stiff plate having a high density since the water is held in the materials. In preparing a gypsum board reinforced by ordinary fibrous material, glass fiber having a maximum fiber length of 10 mm can be incorporated only in an amount of 1%. In the preparation of an asbestos-board, asbestos can be mixed in an amount of 35% at the most, but the maximum fiber length of the asbestos which can be employed is about 5 mm.
As mentioned above, according to the conventional process, it is difficult (a) to uniformly disperse fibrous material in cement-like material; (b) to increase the amount of fibrous material incorporated; (c) to use fibrous material having a long fiber length; (d) to protect the fibrous material from being damaged during kneading; (e) to shorten the kneading time; and (f) to minimize the amount of water.
We have found a novel process for producing a dense stiff board of cement-like material reinforced by fibrous material, which overcomes the above-mentioned difficulties of the conventional process.